The United States has stepped up diplomatic pressure on China by accusing its leaders of "enabling" North Korea to start a uranium-enrichment program and to launch attacks on South Korea, a senior U.S. administration official said this weekend.

In response to the North Korean moves and apparent Chinese acquiescence, Washington is moving to redefine its relationship with South Korea and Japan, potentially creating an anti-China bloc in Northeast Asia that officials say they don't want but may need.

In meetings with their Chinese counterparts in Beijing and in Washington since North Korea launched a deadly artillery barrage at a South Korean island on Nov. 23, U.S. officials have charged that China is turning a blind eye to North Korean violations of U.N. Security Council resolutions, international agreements and a 1953 armistice halting the Korean War that China helped to negotiate.

The accusations mark a further deterioration of the tone and direction of the U.S. relationship with Asia's emerging giant and come as both countries prepare for a second summit next month between President Obama and his Chinese counterpart, Hu Jintao.

"The Chinese embrace of North Korea in the last eight months has served to convince North Korea that China has its back and has encouraged it to behave with impunity," said a senior administration official speaking on the condition of anonymity because of the sensitivity of the subject. "We think the Chinese have been enabling North Korea."

The Korean Peninsula, the official added, has catapulted to the "top of the security agenda when President Hu comes here . . . and the Chinese are aware of it."

Obama called Hu on Sunday night to discuss North Korea and urged China to help send a clear message to Pyongyang "that its provocations are unacceptable," the White House said Monday.

"The president emphasized the need for North Korea to halt its provocative behavior and to meet its international obligations," the White House said. "The president condemned the North Korean shelling of a South Korean island on November 23 and its pursuit of a uranium enrichment program in defiance of its obligations." It said Obama "also highlighted the American commitment to the security of our allies in the region."

The White House statement said Obama and Hu agreed on the importance of working together toward the "shared goals" of peace and stability in Northeast Asia and the denuclearization of the Korean Peninsula. But it made no mention of Hu's response to Obama's complaints about North Korea."

A Chinese statement on the phone call said Hu "regrets the loss of lives and property" in the North Korean shelling last month. It was the first such comment from Hu about the attack, but it echoed an earlier North Korean expression of regret for killing civilians.

The U.S. exasperation with China over the Koreas has been evident since June, when Obama accused China of "willful blindness" in remaining silent over North Korea's suspected sinking of a South Korean warship in March. But the administration's position now that China is in effect partially to blame for the problems is new.

At a meeting Monday with the foreign ministers of Japan and South Korea, Secretary of State Hillary Rodham Clinton hopes to begin the process of tightening the three-way relationship, as a response to the persistent North Korean provocations and China's inaction. The United States and South Korea announced Friday the successful renegotiation of their free-trade agreement, which will be as important strategically as it is economically to the U.S. presence in the region.